Image capturing apparatuses such as image scanners, facsimile machines, printers or copiers are widely used for capturing or scanning images of documents. As known, the image capturing apparatus usually has a document feeder for successively and continuously feeding many paper sheets. Typically, the document feeder has a sheet pick-up device in contact with the front edges of the documents so as to feed the paper sheets one by one.
Referring to FIGS. 1A and 1B, schematic cross-sectional views of a sheet pick-up device of a document feeder are respectively illustrated. FIG. 1A schematically illustrates the sheet pick-up device in a feeding mode. FIG. 1B schematically illustrates the sheet pick-up device in a standby mode. The sheet pick-up device 2 is disposed under an upper cover 3. The operation principles of the sheet pick-up device in the feeding mode and the standby mode are illustrated as follows.
As shown in FIG. 1A, when the document feeder 1 is operated in the feeding mode, a driving shaft 21 of the sheet pick-up device 2 is driven by a motor (not shown) to rotate in a direction R1, and a sheet pick-up arm 22 of the sheet pick-up device 2 is lowered to a sheet feeding position. Meanwhile, a sheet pick-up roller 221 of the sheet pick-up arm 22 transports a document S forwardly and a sheet separation roller 222 of the sheet pick-up arm 22 is rotated to separate the top paper sheet from the stack of paper sheets, thereby picking a single paper sheet.
As shown in FIG. 1B, after the document S has been fed across the sheet separation roller 222, the driving shaft 21 is driven by the motor to rotate in a direction R2, the sheet pick-up arm 22 of the sheet pick-up device 2 is lifted to a standby position.
In the above document feeder 1, the sheet pick-up arm 22 is transmitted by the driving shaft 21 to move in either the sheet feeding position or the standby position with the assistance of two one-way spring clutches. Referring to FIG. 1C, a schematic perspective view of the sheet pick-up device 2 is illustrated. The sheet pick-up device 2 of FIG. 1C includes two one-way spring clutches 23A and 23B, which are mounted on the driving shaft 21 and connected to the sheet pick-up arm 22. The operation principles of these two one-way spring clutches 23A and 23B will be illustrated as follows.
First of all, please refer to FIGS. 1A, 1B and 1C. When a sheet feeding operation of the document feeder 1 is initiated, the driving shaft 21 is rotated in the direction R1 by the spring clutch 23A so as to descend the sheet pick-up arm 22 to the sheet feeding position. When the sheet feeding operation is terminated, the driving shaft 21 is rotated in the direction R2 and the loading generated from the sheet pick-up arm 22 is balanced off by the spring clutch 23A. Meanwhile, the sheet pick-up arm 22 is lifted to the standby position.
Please refer to FIGS. 1A, 1B and 1C again. When the sheet pick-up arm 22 is further lifted to touch the upper cover 3, the spring clutch 23B is operated to allow for idle running of the driving shaft 21 in the direction R2, thereby reducing the possibility of uplifting the upper cover 3 by the sheet pick-up arm 22.
A sheet pick-up device which has two one-way spring clutches was disclosed in for example U.S. Pat. No. 6,390,463, the contents of which are hereby incorporated by reference.
Although the approach of mounting two one-way spring clutches 23A and 23B on the driving shaft is advantageous for reducing the possibility of uplifting the upper cover by the sheet pick-up arm, there are still some drawbacks. For example, the extra one-way spring clutch increases the cost of the document feeder 1 as well as the complexity of the sheet pick-up device 2.
In views of the above-described disadvantages resulted from the prior art, the applicant keeps on carving unflaggingly to develop an improved sheet pick-up device according to the present invention through wholehearted experience and research.